Biogas, also referred to as bio-methane, swamp gas, landfill gas, and digester gas, is the product of anaerobic digestion, e.g., the decomposition of waste material without the presence of oxygen which yields predominantly methane and carbon dioxide. After proper processing to appropriate purity, captured biogas is readily usable as a green/renewable fuel for facility heating, electrical power co-generation, and as vehicle fuel for natural gas powered vehicles. Use of such captured “waste” gas as fuel to power electrical generators, charge fuel cells, and the like, is becoming increasingly more common as the economics for the capital expenditures spurred by environmental concerns, recycling, and use of green energy sources are now justified. However, to date, adequate technology addressing the sample conditioning of biogas extracted from the typical low pressure (<1 psig) sources such as biodigesters does not exist.
Several generation and collection sources of biogas exist across a wide range of disciplines, i.e., waste water treatment, solid waste/land fill disposal and management, food processing plants, and the agricultural industry, including processing farm animal waste.
Probably one of the most common and familiar biogas generation and collection sources is from landfills. Advances have been made to promote biogas generation from those sources. For example, one process involves locating a system of pipes in the landfill to inject air into select land fill strata, effectively comprising an in situ digester/bio reactor. The injected air increases the decomposition rate of the land fill solids with a resulting increase in the production rate of methane containing biogas.
The quality and quantity of generated biogas, as would be expected, is based part, on the nature of its source. For example, studies have shown biogas production from a small landfill would be expected to produce over a million standard cubic foot per day (1,238,000 scfd or 35 cmd) for approximately 20 years. Waste water plants (sewage) produce 1.0 cf of digester gas per 100 gallons of waste water per person (2.8 cm). In the agricultural field, a single dairy cow generates the prodigious quantity of 100 cf of digester gas per day (2.8 cmd).
As noted above, anaerobic digestion (bacterial digestion carried out in the absence of oxygen) produces biogases typically from in situ digestion (e.g., landfills) or anaerobic digester systems. Biogases generated from anaerobic digestion contain a mixture of burnable hydrocarbon gas, (methane), VOC's, hydrogen sulfide, siloxanes including Volatile Methyl Siloxanes (VMS), water, and water vapor. A cubic foot of methane has an energy capacity of 1020 BTU (˜36,000/cm). Therefore, when a biogas stream is composed of 50 to 60% methane, the heat value of that gas approximates 500-600 BTU/lcf (up to ˜24,000/cm).
Before it can be used effectively as a fuel source, however, biogas must be processed. Such processing requires removal and/or minimization of typical impurities found in the biogas output stream. The cleaning begins with particulate removal, followed by removal of water, and, when the desired end product is intended to provide a high quality gas stream, H2S, sulfur species, siloxanes, CO2, digestion generated VOCs (Volatile Organic Chemicals) and oxygen content. The resulting gas is blown, under pressure through a suitable conduit, e.g., 18 inch pipe, to a storage container or directly to a utilization source (heat furnace, fuel cell, etc.). Subject to required purity standards, the resulting cleaned gas may also be utilized independently, blended with local pipeline gas, and even pressurized for use as CNG (Compressed Natural Gas) for powering vehicles.
The level and degree of processing of biogas can also vary based on the intended use of the biogas. For example, if nothing more, entrained water must be removed before biogas can be burned, and the effective removal thereof confirmed by sensor based-analysis. More modern designs of digesters/bioreactors produce biogas with reduced VOCs but increased H2S in the stream. Removal of the H2S “pollutant” is critical to use of the biogas in all applications except plant heating. Another newer technology involves conversion of biological material in waste water to electricity via a microbial fuel cell. Due to the sensitive chemistry involved in fuel cells, higher gas purity and therefore, a more rigorous level of biogas cleaning is necessary prior to being used to power the fuel cells.
If intended for use in combustion engines, substantial removal of Hydrogen Sulfide and siloxanes from the biogas is necessary and operationally critical. Failure to achieve such siloxane removal leads to damage resulting from formation of silica-based deposits and coatings on engine components. Effective removal thereof requires confirmation by sensor based-analysis.
Higher purity requirements can also be found in the use of biogas qualified for blending with other pipeline gases and or as compressed natural gas (CNG). In such cases, the gas must be dried and Carbon Dioxide removed. The highest purity requirements are established by standards such as ISO 15404-2006 in where, in addition to the above scrubbing steps, the removal of all moisture is necessary and the gas compressed to high pressures (6,000 psig) for use as Natural Gas Vehicle (NGV) fuel or compressed natural gas (CNG). The success of such processing must again be confirmed through sample takeoff and analysis.
Consequently for industrial biogas utilization, systems sensor/analyzer technology must be used to confirm to measure the composition of the feed gas and processed gas and to confirm that the cleaning steps have been successful.